The utilization of various polymers has been studied in attempts to control the pharmacokinetics of drugs, such as the sustained release, targeting, and prolonged blood residence (stealth effect) of drugs. Of these polymers, biodegradable polymers which are degraded in vivo need not be removed after their introduction in vivo, and thus they are used commonly. Well known biodegradable polymers in practical use include polylactic acid (PLA), polyglycolic acid (PGA), and lactic acid-glycolic acid copolymer (PLGA). These polymers and copolymers have degradation rates controllable by copolymerization ratio, molecular weight, etc., and they are suitable as carriers of hydrophobic drugs. However, when they are formed into injectable fine particles (microspheres or nanospheres), they are prone to agglomerate in aqueous solutions because of their hydrophobicity, thus posing a problem with their dispersibility in administered solutions or in the body after administration. They also present problems with biocompatibility, such as subcutaneous inflammation or tissue blackening.
In arthropathies, such as osteoarthritis and rheumatoid arthritis, synovitis occurs and frequently becomes the cause of pain. Thus, treatment methods are attempted which administer drugs, such as steroidal or nonsteroidal antiinflammatory agents, by the oral route, or directly inject these drugs into the articular cavity. However, clearance in the articular cavity is so rapid (see non-patent document 1 and non-patent document 2) that it is difficult to sustain the effective pharmacodynamic concentration following a single dose, thereby suppressing inflammation for a long term. Thus, there are reports of various attempts at slow release of drugs, such as antiinflammatory agents, by encapsulating them in base materials. For example, reports have been issued on liposome (see non-patent document 3 and non-patent document 4), albumin microspheres (see non-patent document 5), gelatin/chondroitin sulfate microspheres (see non-patent document 6), and lactic acid-glycolic acid copolymer (PLGA) microspheres (see non-patent document 7).
However, such base materials per se are also known to cause pain (crystal-induced pain) (see non-patent document 8). The cause of this pain is unknown, but may be associated with the size or biocompatibility of microspheres. There is also a report of using PLGA nanospheres decreased in size (see non-patent document 9), but the problem of biocompatibility remains unsolved.
Many reports have been presented on drug carriers using biopolymers, such as polysaccharides, collagen, and gelatin. Since they are hydrophilic materials, however, they are not suitable for supporting hydrophobic drugs, and they are inferior to PLA, PGA, PCL and PLGA in terms of the encapsulation efficiency and the duration of slow release. There is a report, for example, of nanospheres using a compound comprising dextran and PLA bonded together (see non-patent document 10 and non-patent document 11). However, PLA is bonded to dextran by ester linkage, so that in vivo hydrolysis of the junction is prompt (1 day or so). Since the compound is a dextran-based material, moreover, it is not suitable for the purposes of targeting and slow release of drugs.
On the other hand, hyaluronic acid (HA) is a biomaterial (polysaccharide) isolated from the vitreous body of the bovine eye by K. Meyer in 1934, and has been known since olden days as a main component of the extracellular matrix. HA is a glucosamidoglycan composed of disaccharide units, each having D-glucuronic acid and N-acetylglucosamine linked by β(1→3)glycosidic linkage. No species difference exists in the chemical or physical structure of HA, and humans have a metabolic system for HA. HA is the safest biomaterial from the aspects of immunity and toxicity. For example, HA is one of the main components of the joint fluid, and shows an analgesic effect in the joint by its viscoelastic effect and antiinflammatory effect. Actually, drugs having HA as an active ingredient have already been marketed and used as drugs for treatment of arthropathy, such as osteoarthritis or rheumatoid arthritis (for example, Suvenyl (trade name): manufactured and sold by CHUGAI PHARMACEUTICAL CO., LTD.).
There are many reports that hydrogels and microspheres comprising HA or its derivatives were applied to sustained release of drugs (see, for example, non-patent document 12, non-patent document 13, patent document 1, patent document 2, patent document 3, and patent document 4). However, their drug release periods are several days or shorter at the longest, and such materials have not reached the level of practical use.
As HA modification products comprising HA and biodegradable polymers in combination, reports have been presented of biomaterials and pharmaceutical sustained release preparations, such as films comprising HA coated with PLGA (see patent document 5), injections comprising PLGA microspheres and HA mixed together (see patent document 6), protein sustained release preparations having PLGA microspheres dispersed in HA hydrogel (see patent document 7), HA coated nanospheres by electrostatic interaction of polycaprolactone (PCL) using a cationic surface active agent, and HA-PLL having HA bonded to a poly-L-lysine side chain (see non-patent document 14).
There are also reports of polysaccharide derivatives comprising polysaccharides grafted to biodegradable polymers, such as PLA, PGA, PCL and PLGA, and fine particles using the polysaccharide derivatives. Among them, examples using HA as the polysaccharide are also disclosed (see patent document 8). Specifically, however, there is only a showing of those comprising PCL ester-linked to HA, and the functions of the resulting HA derivatives are not shown at all.
As described above, HA modification products, which comprise HA and PLA, PGA or PLGA grafted together and which are excellent in all of safety, biodegradability and in vivo stability, are not specifically known. Nor were the methods of their preparation known. Moreover, conventional drug carriers were problematical in the encapsulation efficiency of drugs (especially, lower molecular drugs), sustained release period, dispersibility in aqueous solutions, blood residence period, or safety (occurrence of inflammation in vivo, etc.). Furthermore, drug carriers comprising injectable fine particles with minimal agglomeration between the particles and having excellent biocompatibility were not known.    Patent document 1: International unexamined publication WO98/43664    Patent document 2: International unexamined publication WO00/78356    Patent document 3: Officially Published Patent Gazette 1999-513047    Patent document 4: Officially Published Patent Gazette 2003-525232    Patent document 5: Japanese Patent Application Laid-Open No. 1996-208706    Patent document 6: International unexamined publication WO01/28591    Patent document 7: International unexamined publication WO97/13502    Patent document 8: International unexamined publication WO01/88019    Non-patent document 1: Eur. J. Clin. PHArmacol. 42, 301-305(1992)    Non-patent document 2: Clin. PHArmacol. Ther. 39, 313-317(1986)    Non-patent document 3: Biochem. J. 158, 473-476(1976)    Non-patent document 4: J. PHArm. PHArmaco. 45, 576-578(1993)    Non-patent document 5: Int. J. PHArmcol. 39, 129-136(1987)    Non-patent document 6: Arthritis Rhem. 41, 21. 85-2195(1998)    Non-patent document 7: Int. J. PHArm. 195, 179-188(2000)    Non-patent document 8: J. Joint Surgery 11, 87-95(1992)    Non-patent document 9: PHArm. Res. 19, 403-410(2002)    Non-patent document 10: J. Biomed. Mater. Res. 50, 557-565(2000)    Non-patent document 11: PHArm. Res. 20, 1284-1292(2003)    Non-patent document 12: Drug Dev. Ind. PHArm. 25(1), 15-20(1999)    Non-patent document 13: Intern. J. PHArm. 87, 21-29(1992)    Non-patent document 14: Bioconj. Chem. 9, 476-481(1998)